The present invention relates to a boiling water reactor provided with a core structure, incore structural machineries and a recirculation system of a reactor for realizing a reactor capable of improving maintenace workings and operability of the reactor and remarkably reducing labour, load or the like of workers or operators for periodical inspections of the reactor.
A boiling water reactor is a reactor of the type in which slightly enriched uranium is used as a fuel, which is directly boiled in a core by utilizing water as a moderator or coolant and steam is then generated.
A boiling water reactor power plant is generally composed of a reactor system and a turbine system. The reactor system comprises a reactor primary series system including a reactor body having a core fuel, an incore structure and a pressure vessel, a recirculation system, a control rod driving system and a main steam system and also comprises a reactor auxiliary system including an emergency core cooling system.
In a known technology, there has been proposed an improved boiling water reactor in which an internal pump system is utilized for a coolant recirculation system instead of an incore jet pump system. Such a boiling water reactor is shown for example in FIG. 20.
Referring to FIG. 20, a core 2 is disposed at a portion slightly lower from a central portion in the reactor pressure vessel 1. A plurality of control rod guide tubes 3 are arranged below the core 2, and the core 2 is composed of a shroud 4 having an upper opening closed by a shroud head 5. Stand pipes 7 of a centrifugal separator 6 stand from the shroud head 5 and six rectangular flat type driers 8 are mounted on the centrifugal separator 6.
A control rod driving mechanism 9 for driving cross-blade type control rods under guidance of inner surfaces of the control rod guide tubes 3 is disposed at a lower portion of the reactor pressure vessel 1. A plurality of internal pumps 10 are mounted to a bottom portion of the reactor pressure vessel 1 at portions between an inside of the reactor pressure vessel 1 and an outside of the shroud 4.
The core is composed of a plurality of fuel assemblies arranged in a lattice structure, and in each of fuel assemblies, fuel rods in 8-row.times.8-line arrangement are supported by upper and lower tie plates and spacers. An entire structure of the fuel assemblies is surrounded by a channel box. Each of the fuel rods is formed by baking a slightly enriched uranium in the shape of a pellet which is then charged into a fuel clad.
The control rod has a cross shape and acts to control a chain reaction in a fission, and the control rod is charged or inserted into the lattice arrangement of the fuel assembly from the lower side of the reactor pressure vessel 1, and the insertion or withdrawal of the control rod from the fuel assembly is performed by means of the control rod driving mechanism 9 connected to the control rod.
In the core 2, the lower portions of the fuel assemblies are supported by a core support plate 11, the upper portions thereof are supported by an upper grid plate 12, and the entire structure thereof is surrounded by the shroud 4. A main steam pipe 13 is connected to an upper side wall portion of the reactor pressure vessel 1, and the steam dried by the driers 8 is transferred to a turbine through the main steam pipe 13. A water supply pipe 14 is also connected to the side wall of the reactor pressure vessel 1 for supplying the coolant into the reactor pressure vessel 1, and the coolant fed thereinto is forcibly circulated by the internal pumps 10.
The boiled two-phase, water and steam, flow from the core 2 is separated by the centrifugal separator 6 into water and steam and the water content in the separated steam is further removed by the driers 8.
The reactor pressure vessel 1 is fixedly mounted on a pedestal 16 through a supporting skirt 15. The attaching or detaching operation of the contol rod driving mechanism 9 is carried out in the pedestal by means of a control rod handling machine 17. An upper end opening of the pressure vessel 1 is pressure-tightly closed by an upper cover 18 and the entire structure of the reactor pressure vessel 1 is accommodated in the reactor containment vessel 19.
With the nuclear power plant including the boiling water reactor of the structure described above, at a time of maintenance operation for the periodical inspection of the power plant, operators or workers enter the lower portion of the reactor pressure vessel 1 for removing the internal pumps 10 and the control rod driving mechanism 9. Although the internal pumps 10 are removed by a removing apparatus, the operators must perform preliminary removing work before the operation of the removing apparatus.
During such preliminary work removing, however, there is a fear that the water coolant will down to the workers from the reactor pressure vessel 1. Under this dangerous environment, the long time working in such place of relatively high possibility of exposure of radiation dose is not desired for the workers in their physical and mental conditions. These may be also referred to for the inspection working or exchanging working of the control rod driving mechanism 9 and incore neutron detectors.
One object of the present invention is to completely eliminate such dangerous maintenance working under the reactor pressure vessel 1.
In another point of view, it is necessary for the operator to pay his highest attention to a water level in the core during the running operation of the reactor. In an ordinary operation, the water level in the core is automatically maintained to a predetermined level by an automatic controlling, and the control of the water level can be usually done by monitoring a display on a control board in a central operation room.
However, in a case of a turbine trip or in a case where the coolant in the core changes into steam and then flows out of the core by an operation of safety valve for a main steam escape after the closing of a main steam isolation valve, the water level in the core lowers downward, and at this moment, a water supply pump, which is driven in isolation by a steam turbine, is operated to thereby start the water supply, but if this starting of the water supply is delayed, the water level in the core further lowers and an emergency core water supply system starts to operate.
The emergency core water supply system operates itself automatically under a preliminarily designed safety control mode of a safety system, but such operation gives feel of strain or pressure to the operaters. On the contrary, when the core water level rises during the operation of the reactor by any accident of, for example, a water level setter, water flows towards the turbine, which may result in damage to the turbine blades.
As described, a transition phenomenon in which undesired water level change is caused gives mental strain to the operators, so that a second object of the present invention is to realize a boiling water reactor having a wide allowable range against the change of the core water level.